Chemical exchange saturation transfer MR imaging of articular cartilage glycosaminoglycans at 3T: Accuracy of B₀ Field Inhomogeneity corrections with gradient echo method

Glycosaminoglycan Chemical Exchange Saturation Transfer (gagCEST) is an important molecular MRI methodology developed to assess changes in cartilage GAG concentrations. The correction for B₀ field inhomogeneity is technically crucial in gagCEST imaging. This study evaluates the accuracy of the B₀ estimation determined by the dual gradient echo method and the effect on gagCEST measurements. The results were compared with those from the commonly used z-spectrum method. Eleven knee patients and three healthy volunteers were scanned. Dual gradient echo B₀ maps with different {increment}TE values (1, 2, 4, 8, and 10ms) were acquired. The asymmetry of the magnetization transfer ratio at 1ppm offset referred to the bulk water frequency, MTR_asym(1ppm), was used to quantify cartilage GAG levels. The B₀ shifts for all knee patients using the z-spectrum and dual gradient echo methods are strongly correlated for all {increment}TE values used (r=0.997 to 0.786, corresponding to {increment}TE=10 to 1ms). The corrected MTR_asym(1ppm) values using the z-spectrum method (1.34%±0.74%) highly agree only with those using the dual gradient echo methods with {increment}TE=10ms (1.72%±0.80%; r=0.924) and 8ms (1.50%±0.82%; r=0.712). The dual gradient echo method with longer {increment}TE values (more than 8ms) has an excellent correlation with the z-spectrum method for gagCEST imaging at 3T.